1. Field of the Invention
The present invention relates to a process for preparing cadmium sulfide for electrophotography and the cadmium sulfide thus produced.
2. Description of the Prior Art
Various electrophotographic photosensitive members are known. Representative electrophotographic photosensitive members are a photosensitive member composed of a support member and a photoconductive layer and a photosensitive member composed of a support member, a photoconductive layer overlying the support member and an insulating layer overlying the photoconductive layer.
The former is widely used for most popular electrophotographic processes comprising charging, imagewise exposure and development, and if desired, additionally transferring the developed images to image receiving members.
The insulating layer in the latter is provided for the purpose of protecting the photoconductive layer, improving mechanical strength of the photosensitive member, improving dark decay characteristics, using the photosensitive member for a particular electrophotographic process, or preventing pollution.
Representative electrophotographic photosensitive members of the latter type and process employing said members are shown, for example, in U.S. Pat. No. 2,860,048, Japanese Patent Publication No. 16429/1966, Japanese Patent Publication No. 15446/1963 (U.S. Pat. No. 3,146,145), Japanese Patent Publication No. 3713/1971 (U.S. Pat. No. 3,607,258), Japanese Patent Publication No. 23910/1967 (U.S. Pat. No. 3,666,363), Japanese Patent Publication No. 24748/1968 (U.S. Pat. No. 3,734,609), Japanese Patent Publication No. 19747/1967 (U.S. Pat. No. 3,457,070), and Japanese Patent Publication No. 4121/1961 (U.S. Pat. No. 3,124,456).
One important factor affecting characteristics of electrophotographic photosensitive members is the photoconductive material. Cadmium sulfide particles is often used as a photoconductive material for electrophotographic photosensitive members. Cadmium sulfide particles possessing desired characteristics may be produced by appropriately controlling amount of doping impurities, conditions for precipitation, firing conditions for diffusing impurities further, and post-treatment conditions.
Electric resistance of photosensitive member comprising cadmium sulfide is affected by the storing conditions of the photosensitive member, that is, whether the photosensitive member is exposed to light or not, and further photodecay properties of the photosensitive member are also varied.
Such changes of electrophotographic characteristics of photoconductive layers attributable to the difference in the storing conditions adversely affect the formation of stable images. In other words, when a photosensitive member has been stood in the dark and then used for copying, for example, a copying machine is not used for a long period of time and then, is operated to produce copies, the resulting image quality at the beginning of copying is different from the image quality obtained by using a photosensitive member which has been stood at a light place, for example, a copying machine is operated for a short time to produce a number of copied sheets and then a copy is made by the copying machine. As the result, it is not possible to produce images of a uniform image quality. Therefore, it is desired to produce images stably without variation of image quality regardless of difference in storing conditions.
As the result of recent improvement in electrophotographic techniques, copying speed has been is increased to a great extent and therefore, it is required that photodecay of photosensitive members is very rapid. For example, it is required that any effect of projection of light in the first cycle of an image forming process comprising imagewise exposure, blanket exposure, and pre-exposure to eliminate the previously formed electrostatic latent images does not remain upon carrying out the second cycle.
In the case of cadmium sulfide used for electrophotographic photosensitive members, the stable formability of images of uniform quality and speed characteristic of photodecay are liable to be lowered when heated at a temperature higher than 100.degree. C.
For example, when cadmium sulfide is dispersed in a thermosetting resin binder followed by thermosetting to produce a photoconductive layer or further an insulating layer is formed on the photoconductive layer by thermosetting and drying, the stable formability of images of uniform quality and speed characteristic are deteriorated by the heat applied.
The deterioration appears to be due to change of Cd-S bond on the particle surface of cadmium sulfide caused by heat and the resulting surface defects.
U.S. Pat. No. 2,876,202 discloses a method for preparing cadmium sulfide for photocell which comprises firing CdS in sulfur gas. However, the reaction efficiency of sulfur with CdS is poor and it is difficult to quantitatively bring sulfur into contact with cadmium sulfide. Therefore, excess or less sulfur is attached to cadmium so that it is difficult to obtain desired characteristics of cadmium sulfide and further sulfur gas is often reacted with oxygen to form sulfur dioxide which adversely affects cadmium sulfide.